In an optical fiber, light is guided by total internal reflection (TIR) in a transparent core surrounded by a transparent cladding of lower index of refraction. Under ideal lossless conditions, light can be guided with nearly perfect efficiency. In practice, however, fibers can be affected with a variety of more or less severe losses.
At least three major loss mechanisms that reduce transmission efficiency have been identified: 1) absorption, resulting from impurities in the core or cladding, causes an exponential reduction in transmission with propagation distance; 2) Rayleigh scattering caused by localized variations in the refractive index of the core and cladding; and 3) imperfections in the core/cladding interface. Substantial progress has been made in improving fiber performance by reducing or minimizing such losses. These improvements have resulted in broadened applications for optical fibers. For example, optical fibers have become attractive as laser oscillators and amplifiers. Such devices combine the excellent properties of laser materials with the high energy confinement available in optical fibers. In particular, single mode fibers have been adapted as optical fiber lasers for a variety of system applications. Such fibers exhibit large energy conversion efficiencies and have excellent coupling properties to single mode transmission fibers, and therefore, result in a high power laser that has numerous important applications, such as in telecommunications fiber transmission systems and networks, as well as other optical fiber communications systems.
As with other types of lasers, the performance of optical fiber lasers is measured in terms of slope efficiency. The ultimate performance of a fiber laser device having a known quantum efficiency, i.e. the maximum probability that a photon of a particular wavelength produced by a pump source will be converted to laser light at the wavelength of interest, is related to the slope efficiency. The slope efficiency is the efficiency with which pump radiation can be absorbed and converted to useful laser light by the active material in the fiber core. Snitzer et al., U.S. Pat. No. 4,815,079, issued on Mar. 21, 1989, for example, discloses a fiber configuration which attempts to efficiently couple radiation to an active single mode core enclosed within a relatively large multimode cladding which, in turn, is surrounded by a light confining outer cladding.
An optical fiber laser is typically pumped from the end of the fiber, but such fibers have also been pumped from the side. There are certain problems that arise when attempting to side-pump a wound pack of optical fiber. For example, as explained in more detail below, the outer protective buffer on the fiber typically must be removed to allow access for the pump energy to the fiber cladding surrounding the core. Removing the buffer exposes the fiber to damage which degrades the reliability of the wound fiber pack. Moreover, if the fiber is wound without the buffer it will likely be damaged during the winding process. As a result of these and other problems heretofore encountered when attempting to side-pump an optical fiber laser in the form of a wound pack, such packs are typically pumped only at the ends of the fiber.
End-pumping an optical fiber has numerous disadvantages, particularly when the pump source is a multispacial mode pump source, such as a semiconductor laser bar or the like. One of the properties of a light source is its radiance which is defined as the amount of light per unit area per unit solid angle that is emitted therefrom. A series of lenses can be used, for example, to change the area of the light beam, but in doing so, the solid angle that the light goes into is reduced. A laser has a very high radiance. However, semiconductor lasers that are used to pump fiber lasers do not have as high a radiance as other laser sources can achieve in accordance with fundamental principles. More particularly, semiconductor lasers are generally provided in the form of an array of semiconductor laser elements, wherein there is a space between each of the lasing regions defining the array. The extra area resulting from the spaces in the array reduces the radiance of the light emitted therefrom. As a result, it is difficult to get light from several different semiconductor lasers into the end of the fiber, due to the relatively low radiance and the limited available cladding area at the end of the fiber. In other words, while semiconductor laser bars are very efficient sources of power, the disadvantage is that they are incoherent. Thus, in order to end-pump with such semiconductor lasers, the laser elements or diodes must be run very hard, i.e. with a high current, which significantly reduces the lifetime of the semiconductor laser. One problem currently being experienced in connection with semiconductor pumps for fiber lasers is that the pumps have a relatively short useful lifetime, due to degradation resulting from operation at such high levels to achieve the desired radiance therefrom. In addition, rather complicated optical arrangements and focusing schemes are used in order to deliver the light to the end of the fiber from all of the diodes in the semiconductor array. For telecommunications applications, hundreds of thousands of hours of useful lifetime is desired from such pumps, but currently available pumps only have a lifetime of approximately seven to ten thousand hours of useful lifetime when operated at such high levels. High current causes various kinds of migration of the material comprising the semiconductor laser and other phenomenon which degrade the device and eventually result in the failure thereof.
Optical fiber made by a process known as the sol-gel process results in a glass that has a lesser density than the usual bulk glass. Sol-gel glass is an optically transparent amorphous silica or silicate material produced by forming interconnections in a network of colloidal, submicrometer particles under increasing viscosity until the network becomes completely rigid, and having space between the particles, and thus having less density than the bulk material. Fibers of pure SiO.sub.2 have been made by the sol-gel process at Ceram Optics in which the center of the fiber comprising the core has a higher density, and thus a higher refractive index, than the outside layer comprising the fiber cladding. The cladding in this type of fiber is a porous glass matrix, such as sol-gel glass, and the pores in the cladding lower the density and, therefore, the refractive index of the glass and enables light to be trapped in the core of the fiber. Fibers made using a porous glass matrix are disclosed, for example, in the patent to Macedo et al, U.S. Pat. No. 3,938,974, which patent is hereby incorporated by reference herein.
A known advantage of the sol-gel fiber is that it is all quartz, and the optical fiber can be made without the need for using conventional dopants, such as germanium, to achieve a higher refractive index in the fiber core to trap the light therein. This is advantageous because the use of dopants can be problematic in a radiation environment. More particularly, dopants may react with radiation and cause the fiber to become light absorbing, an obviously undesirable feature for an optical fiber in a communication or sensing application.
On the other hand, a disadvantage of the sol-gel fiber is that it does not cleave in a clean plane, as does the usual SiO.sub.2 (fused quartz) fiber. The failure to cleave in a clean plane is caused by the fact that very minute pores in the quartz interfere with the usual crack propagating mechanism. A very small flaw in a fuzed quartz fiber can easily propagate in the presence of strain and water vapor. The crack propagation process is inhibited in the porous sol-gel fiber. Therefore, the sol-gel fiber is more rugged and cracks will not propagate and break the fiber until higher stresses are applied.
Generally, optical fibers are made such that they include an outer plastic buffer which is applied immediately or in-line during the fiber manufacturing process. More particularly, when a fiber preform is being drawn using a furnace, the plastic buffer is applied to the fiber and heat treated in-line in order to protect the fiber. In order to avoid any scratches or minute cracks on the cladding, the buffer must be applied immediately and in a very clean environment. Touching the fiber without the buffer weakens the fiber, due to the fact that minute scratches or cracks are formed that will propagate through the typical SiO.sub.2 fiber and eventually cause the fiber to crack and break, particularly during a fiber handling process, such as winding the fiber on a mandrel to form a wound pack of fiber. Thus, if the buffer is removed from the fiber, such as by hot nitrogen or other gases, as is often necessary to enable splicing of fibers, the cladding must be kept very clean and handling of the fiber must be avoided or minimized until the splicing is complete and the buffer is reconstituted over the cladding. Known but expensive techniques and machines exist to prevent the occurrence of scratches on the cladding and to enable such splicing to be performed with minimum damage to the fiber. However, if scratches on the fiber do occur, then the integrity of the fiber is compromised and handling the fiber in the course of winding the fiber to form the wound pack, for example, will eventually cause the fiber to crack.